We
know that God pronounced the coming judgment of the flood 120 years before it happened.Genesis 6:3 says, And the LORD said, My
spirit shall not always strive with man, for that he also is flesh: yet his days shall be
AN HUNDRED AND TWENTY YEARS.That 120
years was not the normal life span at the time, as you can see by looking back at Genesis
chapter 5.The average age to live (excluding
Enoch and Lamech) was 926 years.Even after
the flood, 120 years old was not the average life-expectancy.Noah lived to be 950 years old, which means that he
lived another 349 years after the flood (he was 601 when he got off the ark  Genesis
8:13-16 says, And it came to pass in the SIX HUNDREDTH AND FIRST YEAR, in the first
month, the first day of the month, the waters were dried up from off the earth: and Noah
removed the covering of the ark, and looked, and, behold, the face of the ground was dry.And in the second month, on the seven and twentieth
day of the month, was the earth dried.And God
spake unto Noah, saying,Go forth of the ark,
thou, and thy wife, and thy sons, and thy sons' wives with theeand
he was 950 when he diedGenesis 9:29 says,And all the days of Noah were NINE HUNDRED AND FIFTY YEARS: and
he died.).

So
God gave the pronouncement of the coming judgment upon mankind 120 years before it
happened.The question that arises then is,
Did Noah begin working on the ark immediately, or did God wait for 20 more years
before instructing him to start on it?Apparently,
God in His grace, warned man of the coming judgment, but did not actually give the command
to start building the ark for 20 years.I say,
apparently, because the Bible does not give the exact age of Noah when he received the
commandment; but of when he had Shem, Ham, and Japheth, at which time he was 500 years
old.Genesis 5:32 32 says, And Noah was
five hundred years old: and Noah begat Shem, Ham, and Japheth.

In
chapter 6 and verse 3, God makes His statement about man only having 120 more years to
respond to Him.Then down in verse 10 it
brings up again the matter of Noah having Shem, Ham, and Japheth.Genesis 6:10 says, And Noah begat three sons,
Shem, Ham, and Japheth.So we know that
he was 500 years old at that time.We then
know that Noah was 600 years old when the flood started.Genesis 7:6-7 says, And Noah was SIX HUNDRED YEARS OLD when the flood
of waters was upon the earth.And Noah went
in, and his sons, and his wife, and his sons' wives with him, into the ark, because of the
waters of the flood.

So
here is the chronology: verse 3 says that God is going to give man 120 years before He
sends judgment; Noah is 500 years old down in verse 10 (comparing it with 5:32); and Noah
is 600 years old when the flood starts (chapter 7:6-7).So it seems that God gave the warning 20 years before giving Noah
instruction to actually start building the ark, and it took Noah approximately 100 years
to complete the project.